A rights group says police in the Democratic Republic of Congo have executed dozens of young people during an anti-crime campaign that began a year ago.

U.S.-based Human Rights Watch released a report Tuesday that said "Operation Likofi" resulted in the deaths of at least 51 people and the disappearance of at least 33 others suspected of gang activity. The campaign lasted from November 2013 to February 2014.

Tuesday's report quotes the group's Africa director, Daniel Bekele, calling for Congolese authorities to investigate the killings, beginning with the commander of the operation, and to bring those responsible to justice.

The report says uniformed police dragged suspected gang members known as "kuluna" from their homes at night and shot them dead outside their homes, or in the markets where they slept or worked. The report says many others were taken without warrants and disappeared.

It says the police action initially was used as a warning to others, with the suspects beaten in public places or bodies left where people could see them.

Human Rights Watch says in all the cases it investigated, the victims posed no imminent threat to life that would have justified killing them.

On November 13, the families of 25 victims sent a letter to Congo's national prosecutor, calling for information to be released about their loved ones and demanding the police involved be held responsible.

The letter came a month after the United Nations published a report on the killings, which resulted in Congo expelling the U.N. human rights director, Scott Campbell, stationed in Congo on October 17.